This invention relates to general aspherical surface optical testing; i.e., optical surface that departs significantly from the spherical form.
A perfectly spherical surface as determined by exact calculations can never form a perfect image of finite object by either reflecting or refracting. The described defect which is caused by the geometry of a spherical surface is known as spherical aberration. The rays of light which make the largest angle with the optical axis will be the most influenced by this defect. Spherical aberration may be reduced by use of aspheric surfaces or, in the case of multiple element systems, by taking advantage of a counteracting effect based on errors inherent to each of the different surfaces. The reduction of spherical aberration of an acceptable amount is one of the first problems of the optical designer.
The testing of a concave mirror or a lens has evolved with improvements over the years. An earlier method of testing curved mirrors or lens is known as the Foucault knife-edge test which includes placing a pinhole-source of light at infinite distance behind the lens or at the center of curvature of the mirror (or as near the center as possible). The eye placed at the image of the pinhole should see the entire lens or mirror illuminated. A knife-edge moved across the image immediately in front of the eye will determine where the image is located and will show defects in the lens or mirror by irregular darkening of the image. This test was employed as a customary test by amateur telescope-mirror makers.
Another test method known as the Ronchi test is considered an improved method over the Foucault knife-edge test for testing curved mirrors. The Ronchi test for testing curved mirrors is to replace the knife-edge with a transmission grating, having 40-200 lines to the inch, and to replace the pinhole source with a slit or a section of the same grating.
Mirror and optical surface technology and improvements directed to testing for surface imperfections have been enhanced by laser research and the need for improved mirrors of large diameters for space explorations and continued research and development.
A procedure for testing large diameter mirrors is very expensive, particularly for testing mirrors that depart significantly from a spherical form. These types of mirrors are referred to as aspherical mirrors. As examples, for testing an aspherical mirror of 40 inch diameter with a focal point equal to the diameter of the mirror, the testing with aid of Hindle sphere requires a set up and operation which is very costly. A Hindle sphere is constructed for each type of curved surface to be tested, for example, a parabola shaped mirror requires a 40 inch diameter Hindle sphere of the flat type. A 40 inch diameter hyperbola shaped mirror requires a 45 inch diameter Hindle sphere of the sphere type whereas a 40 inch diameter ellipse shaped mirror requires a 38 inch diameter Hindle sphere also of the sphere type. These methods are not able to test general aspheric mirrors.
Desirable would be an inexpensive device for testing general aspheric mirrors. While conic curves, parabola, hyberb and elipses are expressed by x.sup.2 terms like z* a.times.x.sup.2 and z=x.sup.2 /b +y .sup.2 /c, the general aspheric surface includes higher order terms like x.sup.4, x.sup.6, and even x.sup.8.
Therefore, an object of this invention is to provide a general aspherical surface optical testing device.
A further object of this invention is to provide a general aspherical surface optical testing device which employs a laser as the illuminating source.